1. Field of the Invention
The present invention relates to an ink composition comprising an organic fluorescent substance with specific luminescent properties and, particularly, to an ink composition for latent images, especially an ink composition for preventing forgery of images, to prints containing latent images formed from the ink composition, and to a deciphering method and device based on latent image data on prints whereby information recorded as a latent image is obtained and deciphered.
2. Description of the Related Art
In recent years, numerals, letters, patterns, symbols, marks and the like have been recorded on valuable papers such as paper currency, stock certificates, bonds, postage stamps, exchange checks and the like, ID cards, credit cards and cash cards, or company documents, and products or commodities in order to prevent their forgery, manage their maintenance and facilitate their handling.
An example of known means for achieving this involves coating of inks containing fluorescent substances that are colorless but emit light upon exposure to ultraviolet rays, in order to prevent forgery of valuable papers such as those mentioned above, or to determine the genuineness of the fluorescent substance-coated objects (Japanese Unexamined Patent Publication SHO No. 62-24924, Japanese Unexamined Patent Publication SHO No. 62-50790, Japanese Unexamined Patent Publication HEI No. 10-289287, Japanese Unexamined Patent Publication HEI No. 11-80632).
In such means, however, the light emitted upon exposure to ultraviolet rays is visible light which renders the sections recorded with fluorescent substances visible, and therefore a disadvantage has existed in terms of maintenance management, such as preventing forgery.
Japanese Unexamined Patent Publication SHO No. 61-18231 describes a recording sheet characterized in that a latent image is recorded with a fluorescent powder composed of regularly distributed crystals that emit light in the infrared region upon excitation by infrared rays, and since the latent image is not visible to the naked eye and the fluorescent body used emits light in the infrared range under exposure to infrared rays, it satisfies the desired function of preserving privacy during the procedure of reading the latent image-bearing recording sheet.
However, the fluorescent powder used in this case is an inorganic metal-based fluorescent body which requires high energy for light emission, while stable measurement cannot be achieved due to the low degree of fluorescence, and the hardness of the material renders it poorly resistant to abrasion by parts in the printing device.
Japanese Unexamined Patent Publication No. 2000-129185 describes an invisible ink composition comprising an invisible material which absorbs light in the wavelength range of 650-715 nm and emits light in the wavelength range of 670-720 nm, containing at least one non-complexed invisible metal phthalocyanine fluorescent body, and an ink vehicle including water and at least one organic solvent, wherein invisible images (latent images) are recorded on bond documents with the invisible ink composition to manage their maintenance.
However, the material lacks durability because it employs an invisible dye with an organic solvent and, therefore, when used for purposes that require high durability, such as in paper currency or in documents or articles that experience rough conditions during the treatment or conveying steps, the clarity of detected images has tended to significantly deteriorate, causing a problem for detection and deciphering of the latent images.